History of Mercury Loading in the Upper Mississippi River

Whole-basin Hg accumulation rates increased from 3 kg/yr before European .... when European settlers established a permanent presence near the conflu ...
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Environ. Sci. Technol. 1999, 33, 3297-3302

History of Mercury Loading in the Upper Mississippi River Reconstructed from the Sediments of Lake Pepin S T E V E N J . B A L O G H , * ,† DANIEL R. ENGSTROM,‡ JAMES E. ALMENDINGER,‡ MICHAEL L. MEYER,† AND D. KENT JOHNSON† Metropolitan Council Environmental Services, 2400 Childs Road, St. Paul, Minnesota 55106, and St. Croix Watershed Research Station, Science Museum of Minnesota, Marine on St. Croix, Minnesota 55407

An array of sediment cores was analyzed to determine historical trends in mercury (Hg) accumulation in Lake Pepin, a natural lake on the Upper Mississippi River. Wholebasin Hg accumulation rates increased from 3 kg/yr before European settlement (ca. 1830) to a maximum of 357 kg/ yr in the 1960s. The recent Hg accumulation rate (110 kg/ yr, 1990-1996) is experimentally indistinguishable from measured Hg loadings in the river entering the lake, indicating that accumulation rates in Lake Pepin correspond quantitatively to river loadings. The modern accumulation rate represents a decline of almost 70% from the peak value, reflecting large decreases in Hg inputs to the Mississippi River from industrial and municipal point sources in the metropolitan Minneapolis/St.Paul area upstream. A total of 18.1 t of Hg has been deposited in Lake Pepin since 1800; half of that (9.0 t) was deposited between 1940 and 1970, when regional growth accelerated rapidly but pollution control mechanisms were inadequate. Point sources accounted for approximately 60% of the Hg accumulating in Lake Pepin in the 1960s, but these inputs have been virtually eliminated since then.

Introduction Human activity has greatly altered the biogeochemical cycle of mercury (Hg), resulting in widespread environmental contamination (1, 2). High-temperature processing of ores, the combustion of fossil fuels and waste materials, and other human practices have released large quantities of Hg into the atmosphere, resulting in its dispersion throughout the environment. Long-range atmospheric transport and deposition of anthropogenically derived Hg is responsible for elevated levels of Hg in remote ecosystems (3). Local and regional sources have historically contributed to ambient Hg deposition as well, and recent emissions reductions in the Upper Midwest are evident in reduced loadings to lakes there (4, 5). Mercury is deposited directly on surface waters and is delivered to them indirectly as well via runoff from the watershed. Catchment characteristics (hydrology, climate, * Corresponding author telephone: (651)602-8367; fax: (651)6028215; e-mail: [email protected]. † Metropolitan Council Environmental Services. ‡ Science Museum of Minnesota. 10.1021/es9903328 CCC: $18.00 Published on Web 08/18/1999

 1999 American Chemical Society

soil, land use/land cover, and topography) strongly influence the amounts and speciation of Hg transported to surface waters (6-10). Twenty-five percent of the atmospheric Hg deposition to terrestrial catchments was delivered to headwater lakes in Minnesota and Wisconsin (11), while only 4-9% was delivered from large river basins in the same region (10). Impacts of Hg contamination on aquatic ecosystems vary widely, depending on catchment characteristics and surface water chemistries (12). In watersheds influenced by human activities, direct point source discharges of Hg to surface waters contribute to Hg contamination, overwhelming the atmospheric input in many cases (13-16). Human activities resulting in landscape disturbance (agriculture, logging, urbanization, etc.) also enhance the delivery of Hg to surface waters (5). These diffuse Hg inputs now exceed point source discharges in major rivers in Minnesota (10, 17, 18), but this was probably not always true. Recent declines in industrial Hg use and stricter regulatory controls on discharges to surface waters have significantly reduced point source Hg contributions. Lake sediments provide a stable archive of past and present inputs of Hg to a lake, permitting a reconstruction of the historical record (19). We collected an array of sediment cores from Lake Pepin, a natural lake on the Mississippi River downstream of the Minneapolis-St. Paul, Minnesota (Twin Cities) metropolitan area. Here we present the results for individual cores and whole-basin Hg accumulation rates in Lake Pepin for the past 200 years and analyze the observed trends in the context of the history of the region. Environmental Setting. Lake Pepin is a large natural flood plain lake on the Upper Mississippi River located approximately 80 km downstream from the Twin Cities metropolitan area (Figure 1). The 122 000 km2 watershed of Lake Pepin is drained by the Mississippi River and its two major tributaries in the region, the Minnesota and St. Croix Rivers. Agriculture is the primary land use in the Minnesota River basin, while forests and wetlands make up much of the headwater Mississippi (above the confluence with the Minnesota River) and St. Croix river basins (10). Both precipitation and runoff increase from west to east across the Lake Pepin watershed, and soils vary from fine-grained calcareous mollisols in the Minnesota River basin to sandy alfisols in the St. Croix River basin (20). The diverse characteristics of the three primary sub-watersheds (headwater Mississippi, Minnesota, St. Croix) result in broad differences in the makeup of the water leaving them. Nutrient, suspended sediment, and Hg concentrations and loads are much greater in the Minnesota River than in the headwater Mississippi and St. Croix Rivers (10, 20). The Mississippi River integrates these diverse waters as it flows into Lake Pepin. The hydraulic residence time in Lake Pepin is typically greater than 5 days (21), making the lake an efficient settling basin for suspended sediments. Sediment accumulation rates are greatest at the upper end of the lake and decrease downstream (22-25). Between 85 and 90% of the sediment load originates in the Minnesota River basin, and sediments deposited in the lake are composed primarily of fine silt and clays (25). The lake is an efficient trap for heavy metals discharged from industrial and domestic sources upstream (23).

Methods Sediment Coring and Dating. Twenty-five sediment cores were collected (September 1995 and June and October 1996) along five shore-to-shore transects positioned perpendicular to the flow axis of Lake Pepin (Figure 1). The transects were VOL. 33, NO. 19, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Location of core sites (circles) and depositional regions (shaded areas) for fine-grained sediments in Lake Pepin. Closed circles represent “detailed core” sites. Nondepositional areas (dominated by sand, gravel, and coarser materials) were delineated by seismic reflection profiling and reconnaissance coring. The total depositional area was subdivided into five depositional regions, one for each transect of cores (I-V), by the perpendicular bisector between adjacent transects. Inset map shows major subbasins of the Upper Mississippi River above Lake Pepin. distributed at roughly equal distances from upper to lower reaches of the lake, and five cores were taken along each transect. Cores are designated here by transect number (I-V from upper to lower lake) and position along the transect (1-5 from the Minnesota to the Wisconsin side of the lake). Core locations were recorded in the field by differential GPS. Sediments were collected during the open-water season using piston corers operated from the lake surface by rigid driverods. A surface corer equipped with a 7-cm-diameter polycarbonate core barrel was used to collect a continuous 2-m section of the upper sediments at all coring sites (26). Additional 1-m core sections were taken below the surface core section with a square-rod Livingstone corer (27) at those sites where very rapid sediment accumulation was anticipated (transects I-III); total core length at these locations was 3.5-4 m. Cores were dated and stratigraphically correlated using 210Pb, 137Cs, 14C, magnetic susceptibility, pollen analysis, and loss-on-ignition (25). Lead-210 was measured by R-spectrometry methods (210Po distillation) at 17-24 depth intervals on 10 “detailed cores”, and dates were calculated according to a whole-basin crs model in which core-specific 210Pb inventories from individual cores are integrated across the basin (28). The detailed cores were selected, two from each transect, based on the quality of the magnetic profiles (Figure 1). For all 25 cores, whole-core magnetic susceptibility measurements were made at 2-cm intervals using a Bartington MS2 core logging sensor and automated track. Susceptibility profiles show a striking pattern of peaks and troughs that could be readily correlated among cores within each transect and, for the stronger features, from transect to transect throughout the lake basin. Cesium-137 (20 cores) 3298

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and pollen analysis (together with loss-on-ignition; 10 cores) provided two additional dating markers used to constrain the 210Pb model: the 1964 137Cs peak and an 1860 rise in ragweed pollen. Two AMS 14C dates, obtained on small woody macrofossils (about 10 mg each) in two separate cores, provided estimates of presettlement sedimentation rates in Lake Pepin. The dates, 400 ( 50 14C yr BP (CAMS-39310) and 740 ( 50 14C yr BP (CAMS-39311) occur at sediment depths with magnetic susceptibility features that could be traced among most cores. Radiocarbon dates were calibrated to a calendrical time scale according to Stuiver and Reimer (29). Hg Analysis. Total Hg was determined in 20 freeze-dried stratigraphic sediment sections from each of 10 cores and in top (modern) and bottom (presettlement) sections from each of 10 other cores. Subsamples (0.1 g) were digested in 10 mL of concentrated HNO3:H2SO4 (70:30) at 65 °C for 4 h in rigorously acid leached 60-mL Teflon vials. Samples were cooled, then 6 mL of KMnO4 (5% w/v) and 3 mL of K2S2O8 (5% w/v) were added, and the digestion continued at 65 °C for 2 more hours. Digestate volume was brought to 25 mL with deionized-distilled water, and Hg was determined on an aliquot of the digestate using cold vapor atomic fluorescence spectrometry with single gold trap amalgamation (30). Two different reference materials (BCR-320 river sediment and BCSS-1 marine sediment) were typically analyzed with each core. Our measured mean concentrations of total Hg were 960 ng/g in BCR-320 (n ) 10; relative standard deviation (RSD) ) 9%; certified value ) 1030 ng/g) and 172 ng/g in BCSS-1 (n ) 8; RSD ) 4%; accepted value ) 176 ng/g (31)). Matrix spike recoveries averaged 104% (n ) 4), and the mean RSD of four separate triplicate analyses was 3%. All samples

TABLE 1. Total Hg Concentrations (ng/g) in Lake Pepin Sediment Cores

a

FIGURE 2. Mercury concentration and accumulation flux profiles for 10 210Pb-dated sediment cores analyzed in detail. Cores are arranged from upper to lower lake. had Hg concentrations above our method quantitation limit of 8 ng/g (calculated as 10 times our reagent blank value).

Results and Discussion Mercury concentration profiles in all sediment cores record a history of contamination that began with European settlement in the early 1800s, peaked after World War II, and subsequently declined to current levels (Figure 2). Concentrations of Hg in deep, presettlement samples varied little throughout the lake, ranging from 33 to 40 ng/g. These levels are typical of presettlement sediments from lakes across southern Minnesota (5) and are comparable to surface soil Hg concentrations in the Minnesota River basin (range: 1844 ng/g, n ) 18; Balogh, unpublished data). All cores show increased Hg concentrations soon after 1800, when European settlers established a permanent presence near the confluence of the Minnesota and Mississippi Rivers. Sediment Hg concentrations increased rapidly thereafter, exceeding 200 ng/g throughout the lake by 1900. All cores except those farthest downstream (cores V.1 and V.4) show a brief period

corea

presettlement

peak

modern

I.2 I.3 II.1 II.3 III.2 III.4 IV.2 IV.4 V.1 V.4

33 34 34 33 40 36 38 34 37 39

662 669 528 561 519 503 570 499 452 492

106 120 120 128 121 135 123 128 137 161

Cores arranged from upper to lower lake.

of steady or decreasing Hg concentration between 1920 and 1950. The trend of increasing Hg concentrations was quickly reestablished, however, and peak levels were observed in most cores between 1949 and 1968. Mercury concentrations decreased rapidly in all cores beginning in the late 1960s, and these decreasing trends continued to the time of sampling. Peak Hg concentrations were 3-6 times higher than surface (modern) values, and 12-20 times higher than presettlement levels (Table 1; Figure 2). Peak Hg concentrations decreased in the downstream direction, from 669 ng/g in core I.3 to 452 ng/g in core V.1, but surface concentrations were highest in the downstream cores (V.1 and V.4). Analysis of surficial sediments from transects of Lake Pepin in 1979 yielded Hg concentrations ranging from 153 to 198 ng/g (32). Our data corresponding to this time are generally within this same range, except for the cores from farthest downstream, which show slightly higher (up to 261 ng/g) concentrations. Although Hg concentration trends are tightly synchronous among coresse.g., the increase above background in the mid-1800s and the steep decline in the 1960ssthe sediment depths at which these features appear are much greater in profiles from the upper lake than in the lower lake. For example, the onset of the steep decline in Hg concentrations occurs at 112 and 96 cm in cores I.2 and I.3 (upper lake), respectively, but at 44 and 40 cm in cores V.1 and V.4 (lower lake), respectively. This pattern reflects the strong riverine gradient of decreasing sedimentation rates from upper to lower lake. The fact that Hg profiles are temporally concordant irrespective of core depth provides strong evidence that Hg stratigraphy in Lake Pepin is largely unaffected by postdepositional sedimentary processes (3). Mercury accumulation fluxes (mg m-2 yr-1) for individual cores were calculated as the product of the sediment accumulation fluxes and Hg concentration data for each strata in each core. All cores showed a peak Hg accumulation flux in the 1960s (1.8-3.7 times modern; 50-260 times presettlement), followed by an abrupt decline in the 1970s (Figure 2). Accumulation of Hg was greatest in cores I.2 and I.3 and decreased with distance downstream, reflecting the strong negative gradient in sediment accumulation fluxes with distance downstream (25). Whole-basin Hg accumulation fluxes (mg m-2 yr-1) were calculated by weighting the Hg accumulation flux of each core by the portion of the depositional area it represents and then summing over all cores. To compare and combine data from all the cores, whole-basin fluxes were calculated at decadal intervals from the present to 1930, at 20-year intervals from 1930 to 1890, and at 30-year intervals from 1890 to 1830. An average presettlement flux was calculated for each core from all analyzed strata from before 1830. The wholebasin Hg accumulation flux in Lake Pepin prior to 1830 was 0.03 mg m-2 yr-1 (calculated over the entire 101 km2 lakeVOL. 33, NO. 19, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Whole-basin Hg accumulation rates in Lake Pepin, averaged at decadal intervals. bottom area; Figure 3). Presettlement Hg accumulation fluxes measured in 50 nonriverine lakes throughout Minnesota were typically less than 0.02 mg m-2 yr-1 (based on single-core values that overestimate whole-basin accumulation fluxes; 5). This difference reflects a moderately greater presettlement sediment accumulation flux in Lake Pepin relative to the other lakes. The whole-basin peak Hg accumulation flux in Lake Pepin was 3.53 mg m-2 yr-1, which is approximately 10-100 times greater than the peak levels observed in other lakes in Minnesota (5). Modern Hg accumulation fluxes in Minnesota lakes range from 0.01 to 0.16 mg m-2 yr-1 (5), much lower than the modern whole-basin Hg accumulation flux in Lake Pepin (1.08 mg m-2 yr-1). Modern Hg accumulation fluxes in remote lakes in Finland, Sweden, Canada, the United States, and Russia have been reported in the range of 0.002-0.076 mg m-2 yr-1 (11, 33-39). Both higher sedimentation rates and higher sediment Hg concentrations contribute to higher peak and modern Hg accumulation fluxes in Lake Pepin relative to nonriverine lakes in Minnesota and elsewhere. The ratio of the modern whole-basin Hg accumulation flux in Lake Pepin to the presettlement flux (M:P Hg flux ratio) is 36 (1.08/0.03), much higher than in nonriverine lakes in Minnesota (4, 5, 19) and elsewhere (34, 35, 38, 39) where typical values range from 1 to 4. Increased sediment accumulation is primarily responsible for the large M:P Hg flux ratio in Lake Pepin. The sediment accumulation flux in Lake Pepin increased from 78 mg cm-2 yr-1 prior to settlement to 864 mg cm-2 yr-1 recently (M:P sediment flux ratio ) 864/ 78 ) 11, 25). The increase in basin-averaged sediment Hg concentration from presettlement to modern times can be calculated as the quotient of the M:P Hg flux ratio and the M:P sediment flux ratio (36/11 ) 3.3). (The Hg concentrations arrived at in this way are mass-weighted averages representing the mean Hg concentration of the entire mass of sediment accumulating in the lake.) This M:P Hg concentration ratio for Lake Pepin (3.3) is comparable to those for lakes in the metropolitan Twin Cities area (mean ) 3.4, range ) 1.6-7.4, n ) 19; 5), suggesting that similar factors are responsible for the modern enhancement of Hg concentrations in sediments 3300

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in Lake Pepin and the metropolitan lakes. Increased diffuse Hg inputs (atmospheric deposition, surface runoff, etc.) augment recent sediment Hg concentrations in these lakes relative to presettlement conditions. Point sources now contribute relatively little Hg to area rivers (17, 18) and Lake Pepin and contribute nothing to area lakes. Point sources did contribute significantly to Hg accumulation in Lake Pepin in earlier years. Following an approach similar to that outlined above to calculate the M:P Hg concentration ratio, we can define a peak:presettlement (Pk:P) Hg concentration ratio as the quotient of the Pk:P Hg flux ratio and the sediment flux ratio from the corresponding time. The Hg accumulation flux peaked (3.53 mg m-2 yr-1) in the 1960s, and the Pk:P Hg flux ratio is given by 3.53/0.03 ) 118. The sediment accumulation flux during that decade was 709 mg cm-2 yr-1, yielding a sediment flux ratio of 709/ 78 ) 9.1. The basin-averaged Pk:P Hg concentration ratio is then 118/9.1 ) 13, which is much higher than the M:P Hg concentration ratio in Lake Pepin (3.3). It is also significantly higher than the mean value for metropolitan area lakes (4.8; 5) from the same time, suggesting that different factors were responsible for the enrichment of Lake Pepin sediments in the 1960s than were influencing the sediments in metropolitan area lakes. The primary difference was that the Mississippi River and Lake Pepin were receiving significant point source Hg inputs at this time, whereas the other lakes received none. We can estimate the relative contribution of point sources to Hg loadings in Lake Pepin in the 1960s. If Lake Pepin were receiving no point source Hg inputs, its Pk:P Hg concentration ratio at that time should have been similar to that of the metropolitan area lakes (4.8). Given the same sediment flux ratio (9.1), the estimated Pk:P Hg flux ratio for that time would then have been 4.8 × 9.1 ) 44, and the Hg accumulation flux would have been 44 × 0.03 ) 1.3 mg m-2 yr-1. Thus, point sources increased the Hg accumulation flux in Lake Pepin from 1.3 to 3.53 mg m-2 yr-1. By this estimation, point sources contributed over 60% (100 × (3.53-1.3)/3.53) of the Hg that accumulated in the lake in the 1960s. These calculations also suggest that nonpoint source Hg inputs to Lake Pepin have decreased since the 1960s. The modern Hg accumulation flux (1.08 mg m-2 yr-1) is 17% less than the calculated flux of nonpoint Hg in the 1960s (1.3 mg m-2 yr-1) despite a 22% increase in sediment loading to the lake since the 1960s. The atmospheric flux of Hg to Twin Cities lakes has decreased by approximately 30%, while that to nonmetropolitan area lakes has remained constant (5). It is reasonable that the decrease observed in Lake Pepin falls between these extremes. Whole-basin Hg accumulation rates (kg/yr) were calculated by multiplying the whole-basin Hg accumulation flux for each time interval by the total area of the lake. The variation of the whole-basin Hg accumulation rate over the past 200 years follows the pattern described above for the individual cores (Figure 3). The whole-basin Hg accumulation rate increased above baseline with the advent of European settlement in 1830 and then rose steadily over the next 130 years before peaking (357 kg/yr) in the 1960s. The rate of Hg accumulation in Lake Pepin decreased sharply in the 1970s, and a more gradual decline has occurred since 1980 despite increasing sediment accumulation rates. The recent rate of Hg accumulation (110 kg/yr) remains much higher than the presettlement value (3 kg/yr). The mean annual Hg loading in the Mississippi River at Lock and Dam 3 (UM kilometer 1275, immediately upstream of Lake Pepin) was 102 kg/yr for the years 1995 through 1996 (Balogh, unpublished data). The recent Hg accumulation rate in Lake Pepin (110 kg/yr for 1990-1996) is experimentally indistinguishable from the Hg loading in the Mississippi River as it enters the lake. Lake Pepin is an efficient trap for Hg carried by the river, and both historic and present-day accumulation rates of Hg in Lake

FIGURE 4. Effluent flow and total suspended solids (TSS) load discharged from the Metropolitan Wastewater Treatment Plant, St. Paul, MN (UM kilometer 1345), 1942-1996. Inset shows decadal mean TSS load in kt/yr. Pepin represent quantitatively accurate estimates of contemporaneous Mississippi River loadings. The total accumulation of Hg in Lake Pepin from 1801 to 1996 was calculated by multiplying the annual accumulation rates from Figure 3 by the number of years in each time interval and summing over all the intervals. Over this 196year period, 18.2 t of Hg accumulated in Lake Pepin. The three decades from 1940 to 1970 were the years of greatest Hg accumulation, when 9.0 t (50% of the 196-year total) was deposited. A total of 3.9 t (21%) of Hg was deposited in Lake Pepin between 1970 and 1996, and 5.3 t of Hg (29%) accumulated in the lake during the 140 years from 1801 to 1940. Thus, the total mass of Hg accumulating in Lake Pepin between 1940 and 1970 was 2.3 times that deposited during the 26-year period from 1970 to 1996 and 1.7 times that deposited during the entire 140-year period prior to 1940. Most of the Hg deposited in Lake Pepin over the past two centuries collected in the upper 30% (by surface area) of the lake. The total accumulation of Hg in this section of the lake was determined by multiplying the annual accumulation rates in that section (represented by cores labeled I and II) by the number of years in each time interval and summing over all intervals. The total mass of Hg accumulated in this part of the lake between 1801 and 1996 was 12.5 t, or approximately 69% of the total for the whole lake (18.2 t). Similar calculations for sediment accumulation showed that 39.5 × 106 t, or 63% of the 62.2 × 106 t of sediment accumulated in the lake between 1801 and 1996, was found in the upper 30% of the lake. Strong sediment and Hg accumulation gradients exist in Lake Pepin, with decreasing accumulation in the downstream direction. This record of Hg loadings in the Mississippi River reflects shifts in the ways the river has been viewed and used over the past 200 years and reveals in particular the dramatic results of pollution control efforts over the past 30 years. Both point and nonpoint sources have contributed to the accumulation of Hg in Lake Pepin over the past 200 years. Diffuse landscape disturbances stemming from widespread agriculture, forestry, urban/suburban development, and other human activities have enhanced the delivery of soils and Hg to rivers and lakes in the watershed (5, 10) and are responsible for the rapid accumulation of sediment in Lake Pepin (25). Increased coal combustion and other industrial Hg emissions have resulted in increased atmospheric deposition of Hg to the lake surface and to watershed soils,

enhancing the impact of these nonpoint inputs. The trend of increasing Hg concentrations in sediment deposited soon after settlement through the mid-1900s, however, suggests that other sources have contributed significantly to Hg accumulation in Lake Pepin. Increases in Hg concentrations in Lake Pepin sediments are evident soon after 1800, corresponding to the initial settlement of the area by Europeans. Much of this rise may be attributed to increased atmospheric Hg deposition from local and regional combustion sources, but other direct human uses of Hg might also have contributed. Mercury found widespread use at this time as a medicinal, particularly in the treatment of syphilis and as a purgative (40). Mercury was also used in photography, dentistry, and the tanning and dyeing industries, and mercury thermometers were common. Later, in the 1900s, medicinal use of Hg decreased, but industrial and other uses grew. Prior to 1938, untreated liquid wastes from the growing communities in Minneapolis and St. Paul flowed directly into the river, and water quality suffered. Spring floods cleansed the river on a regular basis until 1917, when the construction of Lock and Dam 1 at St. Paul created a navigational pool from St. Paul to Minneapolis that served as a settling pond for liquid wastes from Minneapolis (41). The Metropolitan Wastewater Treatment Plant, located on the river in St. Paul, commenced operation in 1938, providing primary treatment for much of the domestic and industrial wastewater generated in the region. Water quality in the Mississippi River improved substantially after this; however, regional growth accelerated rapidly over the next 20 years, and wastewater treatment capacity was unable to keep up. This period of rapid regional development and increasingly inadequate wastewater treatment (1940-1970) corresponds with the timing of the most rapid Hg accumulation in Lake Pepin, as described above. Expansion of the Metropolitan Plant and the addition of secondary treatment capability between 1966 and 1968 resulted in vast improvements in river water quality (41). The mean annual load of suspended solids (TSS) discharged from the Metropolitan Plant decreased by 44% between the 1960s and the 1970s (Figure 4). Mercury is almost exclusively associated with suspended solids in both primary and secondary effluent streams at the Metropolitan Plant (Balogh, unpublished data), so reductions in mercury discharges at this time were probably also substantial. Industrial use of Hg decreased after the 1960s, contributing further to declining Hg discharges from the VOL. 33, NO. 19, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Metropolitan Plant and other point sources. The decadal mean whole-basin accumulation rates of Hg in Lake Pepin, reflecting Mississippi River loadings at the time, show a contemporaneous decline of 48% between the 1960s and 1970s (Figure 3). The Clean Water Act (1972) and other pollution control efforts in the 1970s brought further improvements in water quality and further reductions in Hg accumulation in Lake Pepin. The Hg accumulation rate in Lake Pepin decreased through the 1980s and 1990s despite an increasing rate of sediment accumulation. During this time, there were reductions in both point and nonpoint source Hg inputs. In 1994, the annual discharge of Hg from the Metropolitan Plant to the Mississippi River was estimated at 3.7 kg (42). On the basis of data from 1995 and 1996, this discharge was found to represent only about 4% of the annual river Hg loading (18). By the mid-1990s, nonpoint sources of Hg throughout the Lake Pepin watershed contributed the bulk of the Hg deposited in the lake, but as shown here, this was not always the case.

Acknowledgments We thank Scott Schellhaass for assistance with the field work; Kelly Thommes for help with sediment dating and sample handling; Jie Hu for assistance with the Hg analyses; and Cathy Larson for helpful discussions.

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Received for review March 24, 1999. Revised manuscript received July 1, 1999. Accepted July 14, 1999. ES9903328